The present invention is directed to an improved chemical vapor deposited (CVD) silicon carbide (SiC). More specifically, the present invention is directed to a chemical vapor deposited (CVD) silicon carbide (SiC) with reduced stacking faults and high thermal conductivity.
Many manufacturing steps are involved in the production of semiconductor devices. Some steps in the manufacture of semiconductor devices involve rapid thermal processing where heat is necessarily applied and dissipated rapidly from the wafer and wafer holders in the processing of semiconductors. If materials that comprise the wafer and wafer holders have an insufficiently high thermal conductivity, the materials may fail, i.e., crack or fracture due to thermal stress. Additionally, there is a need to manage heat generation and flow in electronic devices. For example, active thermoelectric coolers as well as heat sinks and fans have become ubiquitous in electronic devices such as microprocessors. Electronic devices that contain such “chillers” need to have high thermal conductivities. During operation, many electronic devices generate high amounts of heat. Without suitable thermoelectric coolers, heat sinks, or fans, the electronic devices can not dissipate heat fast enough and they degenerate or break down. Thus such electronic devices would have short life spans, and would be inefficient for industries employing the devices. Accordingly, it is imperative that such electronic devices have heat sinks composed of high thermal conductivity materials to dissipate generated heat.
SiC has been found to be very useful as a material in semiconductor processing equipment and components of electronic devices because of the high theoretical thermal conductivity of SiC crystals. Other inherent properties of SiC that are desirable for such equipment and components are high specific stiffness, strength, hardness, low thermal expansion, chemical and oxidation resistance, and thermal shock resistance. Although SiC has been used in apparatus and electronic devices successfully, there is still a need for a SiC having improved properties. As the computer industry realizes smaller and more advanced semiconductor devices there is a need for more rapid heating and cooling during device fabrication and more surface heating of the wafer during those operations. Thus, apparatus used to make the semiconductors preferably are made of materials that can dissipate heat rapidly. Also, as the speed and memory of electronic devices increases more heat is generated during operation, thus the thermal conductivity of component parts in such devices must increase.
The highest theoretical thermal conductivity for single crystal SiC has been estimated to be about 490 W/mK. However, single crystal SiC is difficult to produce in large enough sizes to employ in the manufacture of such materials as semiconductor furnace support components, and is considerably more expensive than polycrystalline SiC. Unfortunately, polycrystalline SiC does not achieve the theoretical thermal conductivity of a single crystal of SiC due to the presence of grain boundaries, which scatter phonons or sound energy. Accordingly, it is very difficult to predict a range, let alone a specific value, for the thermal conductivity of a chemical vapor deposited, polycrystalline SiC. In addition, point, line and extended defects diminish the thermal conductivity of polycrystalline SiC. Such defects add to the difficulty of preparing a SiC with a desired high thermal conductivity. Another crystalline problem associated with polycrystalline SiC that may effect the thermal conductivity are stacking faults as suggested in W. Kowbel et al. “Effects of Boron Doping on the Thermal Conductivity of Chemical Vapor Infiltration (CVI) SiC”, Journal of Material Synthetic Processes, 4 (1996) pg. 195-204. Kowbel et al. employ both boron doping and annealing at a temperature of 1500° C. to reduce the stacking faults in fiber-reinforced SiC composites (SiC/SiC). Kowbel et al. states that the reduction in the stacking faults improved thermal conductivity of the fiber-reinforced SiC. However, Kowbel et al. only achieved a maximum thermal conductivity of 140 W/mK. Such low thermal conductivities are unsuitable for the demands of the semiconductor device and electronic device industries.
Stacking faults are gaps or separations in the continuity of the crystalline lattice of polycrystalline SiC. If such gaps in the crystalline lattice reduce the capacity of the SiC to conduct and dissipate heat, a “chiller”, such as a heat sink, composed of such material is not reliable in an electronic device where a high heat load is continuously generated. Such reliability of a heat sink to conduct, and dissipate large amounts of heat is becoming more and more important as improved electronic devices are generating very high amounts of energy during operation. Some electronic devices may generate heat load values in excess of 300 Kcals/min. Such high heat generation can readily damage electronic devices after about 200 hours of operation without an appropriate heat sink. Preferably, heat sinks in such devices are composed of materials such as SiC having thermal conductivities of at least 300 W/mK. Most preferably, the thermal conductivity of SiC is at least 375 W/mK. However, with the exceptions discussed below, few if any methods have been developed to provide a SiC with a thermal conductivity that exceeds 300 W/mK, let alone 375 W/mK. Further, improved apparatus employed in manufacturing semiconductors are also generating very high quantities of heat, and require component parts having high thermal conductivities. Accordingly, there is still a need to develop an improved SiC with a thermal conductivity such that the SiC can be employed in apparatus and electrical devices that generate very high beat loads.
Silicon carbide is deposited by CVD from a gaseous mixture of methyltrichlorosilane (MTS), H2, and an inert or non-reactive gas such as argon, helium or nitrogen, argon being preferred. Freestanding SiC is pyrolitically deposited on a mandrel, such as a graphite mandrel, from which it is removable. The MTS is the preferred source of both the silicon and carbon and provides these reactants in stoichiometric (1:1) ratios. The H2 scavenges chlorine to produce hydrochloric acid. The inert or non-reactive gas acts as a carrier gas for MTS (which is liquid at ambient temperatures); and can be varied to adjust velocity of gas flow through the furnace as is necessary to sweep reaction product, such as hydrochloric acid, from the deposited SiC. The inert or non-reactive gas also acts as a diluent, preventing gas-phase reactions that may introduce impurities into the SiC. CVD production of free-standing SiC material are described in U.S. Pat. Nos. 4,990,374; 4,997,678; and 5,071,596, the teachings of these patents being incorporated in their entirety herein by reference. However, the methods described in the foregoing patents did not achieve a SiC with a thermal conductivity of greater than 300 W/mK. Further experimentation was necessary to find a method for obtaining a SiC with a thermal conductivity of 300 W/mK.
U.S. Pat. No. 5,374,412, to Pickering et al. and assigned to CVD, Inc., discloses an impinging flow method to make a polycrystalline SiC with a high thermal conductivity. The thermal conductivity of SiC prepared by the method disclosed in the patent is at least about 300 W/mK. The patent records a thermal conductivity of 304.9 W/mK at 28° C. The SiC is deposited using reactants methyltrichlorsilane (MTS), and H2 gas in an inert carrier gas environment. The conditions included: a deposition chamber pressure of between about 180 and 220 torr, a deposition chamber temperature of between about 1340° C. and 1380° C., a deposition rate of between about 1.0 and about 2.0 μm/min., and an H2/MTS gas partial pressure flow ratio of between about 4 and about 10. Further, H2 supplied as a part of the gas stream, is purified such that H2 contains less than about 1 ppm of O2 gas, and various means are provided to exclude particulate contaminant material from the deposition chamber.
The SiC is fabricated by the aforementioned process on a mandrel that is placed perpendicular to the flow, i.e., an impinging flow configuration, of the reactants. The high thermal conductivity SiC is machined to its end-use configuration, e.g., a hard disc or a read/write head, and highly polished on an appropriate surface or surfaces. The very specific set of deposition parameters set forth above achieve the combination of a high polishabilty and high thermal conductivity SiC.
The '412 patent discloses that thermal conductivity of SiC is dependent on the grain size and purity of the SiC material, i.e., the thermal conductivity increases with increasing grain size and low impurity concentration along the grain boundaries. The grain size, and therefore, the thermal conductivity, is controlled by deposition chamber temperature, pressure and reactant gas flow rates. For example, under conditions of high temperature and low MTS flow rates (which results in low MTS partial pressure), the grain size increases. As the deposition temperature is lowered and the MTS flow increases, the grain size decreases. Thus, altering the aforementioned parameters in CVD of SiC are known to increase the thermal conductivity of SiC. However, because of the crystalline defects in polycrystalline SiC discussed above, altering one or more of the aforementioned parameters does not provide a reliable means of obtaining a SiC having a specific thermal conductivity. For example, decreasing the reactant gas flow rate by half does not increase the thermal conductivity of vapor deposited SiC to a predictable value. The thermal conductivity may increase a few units or only a fraction of a unit. Similar results are true when altering the other parameters described above.
U.S. Pat. No. 5,354,580, to Goela et al. and assigned to CVD Inc. discloses an apparatus and method for CVD deposition of SiC by a parallel deposition process. In the parallel process, the reactant flow is parallel to the mandrel on which the SiC deposits. A deposition chamber in which the flow is parallel to the deposition surface or mandrel provides good potential to obtain high deposition efficiency, and a product of high quality. The SiC produced by the specific process disclosed in the patent had a thermal conductivity of 315 W/mK and a good polishability. The SiC was generated at the following specific conditions: a furnace temperature of 1350° C., a furnace pressure of 200 torr, an argon flow rate of 13 slpm (standard liters per minute measured at atmospheric pressure and 20° C.), H2 flow rate of 22 slpm, and an MTS flow rate of 5.1 slpm. The deposition was performed for 76 hours.
The aforementioned methods have produced polycrystalline silicon carbide having improved thermal conductivities. However, since there is a continuously increasing demand for SiC having very high thermal conductivities, there is a need for a SiC and a method of making the same that has a thermal conductivity of at least 375 W/mK.